Implantable cardiac stimulation devices are well known in the art. Such devices may include, for example, implantable cardiac pacemakers and defibrillators either alone or combined in a common enclosure. The devices are generally implanted in a pectoral region of the chest beneath the skin of a patient within what is known as the subcutaneous pocket. The implantable devices generally function in association with one or more electrodes carrying leads which are implanted within the heart. The electrodes are positioned within the heart for making electrical contact with the muscle tissue of their respective heart chamber. Conductors within the leads connect the electrodes to the device to enable the device to deliver the desired electrical therapy.
Monitoring heart rate at a defined degree of workload is a good parameter to use when monitoring the status of a patient's cardiovascular function and oxygen uptake as well as pacing therapy settings including pacing intervals and pacing sites. Rest rate is often associated with a patient's exercise capacity and health. During hard training the rest rate will decrease as the heart becomes stronger and oxygen uptake and metabolism are improved. The change in rest rate is normally slow and sudden changes depend most often on factors like fever, body posture changes or changes in atmospheric pressure. Cardiac output (CO) is the heart rate (HR) multiplied with the stroke volume (SV).
Regulation of blood flow in the various organs is mainly achieved by narrowing the blood vessels, vasoconstriction, and widening of blood vessels, vasodilation. This tonus of the vascular musculature is influenced by local effects, neural activity and hormonal signals. At rest, most blood vessels are in an intermediate state of tension and relatively constant.
Adaptation of SV to changes in filling and aortic pressure takes place autonomously, by alterations of the end-diastolic resting tension, i.e. the Frank-Starling mechanism. If this regulation is impaired or fails due to suboptimal pacemaker settings, the heart tries to compensate for this by an increased sinus rhythm to maintain the cardiac output. Thus, reduced SV can be associated with suboptimal settings of pacing intervals such as AV delay and VV delay and/or pacing sites. In short term, this will be compensated by an increased heart rate to maintain a sufficient cardiac output. In the long term, the stroke volume may increase towards the initial value due to hypertrophy and heart dilatation and thus the heart rate will decrease again.
The heart rate depends of several factors like activity level, body temperature, atmospheric pressure, stress, vascular resistance and body posture. To monitor changes of the heart rate due to variations of the pacing therapy settings, it is important to have knowledge of these parameters so that the heart rate can be compared at similar or same conditions.
In the art, there have been made significant efforts to develop methods and devices for optimizing pacing therapy settings and/or monitoring diseases based on heart rate.
For example, in US patent application No. 2010/0030295 to Whinnett et al, a method for determining optimal AV delay while pacing at rest heart rate is disclosed. Optimal AV delay is considered to have been found when the highest blood pressure is achieved.
In the international patent application No. WO 2011/008749 to Ternes et al., an implanted device that trends heart rate, both during and without stimulation, and the stimulation changes when average rest heart rate is changed is disclosed.
In U.S. Pat. No. 6,129,744 to Boute, a method for determining heart failure conditions by studying and trending changes in rest rates is disclosed. The result is indicative of the onset of LVD (“Left Ventricular Dysfunction”) and corresponding ejection fraction.
In U.S. Pat. No. 7,3131,439 to Jackson et al., a method for predicting arrhythmias based on trending heart rates is disclosed.
However, there is still a need within the art for improved methods, devices and systems for optimizing cardiac pacemakers.